1. Field of the Invention
This invention relates to a device capable of remotely peening the interior walls of tubes located in areas of limited accessibility. It is particularly useful in rotopeening the heat exchange tubes mounted in the peripheral areas of the tubesheet of a nuclear steam generator in order to relieve tensile stresses in the inside wall of these tubes.
2. Description of the Prior Art
Devices for peening the inside walls of metallic tubes are generally known in the prior art. Such devices are particularly useful in relieving or at least equilibrating the tensile stresses which may be induced across the wall of a metallic tube when that tube is radially expanded, as by a hydraulic mandrel or a cold-rolling tool. Such stress-causing expansions are routinely performed in the heat exchange tubes of nuclear steam generators, particularly in the vicinity of the generator tubesheet, during both the manufacture and the maintenance of the nuclear steam generator. Unfortunately, such stresses can lead to an undesirable phenomenon known as "stress corrosion cracking" in the walls of such tubes if not relieved in some way. In order to fully understand the dangers associated with such stress corrosion cracking, and the utility of the invention in preventing such cracking, some general background as to the structure, operation, and maintenance of nuclear steam generators is necessary.
Nuclear steam generators are comprised of three principal parts, including a secondary side and a tubesheet, as well as a bowl-shaped primary side which circulates water heated from a nuclear reactor. The secondary side of the generator includes a plurality of U-shaped heat exchange tubes, as well as an inlet for admitting a flow of feedwater. The inlet and outlet ends of the U-shaped tubes within the secondary side of the generator are mounted in bores in the tubesheet. The tubesheet hydraulically isolates the primary side of the generator from the secondary side. The primary side in turn includes a divider sheet which hydraulically isolates the inlet ends of the U-shaped tubes from the outlet ends. Hot, radioactive water flowing from the nuclear reactor is admitted into the inlet section of the primary side which contains all the inlet ends of the U-shaped tubes. This hot, radioactive water enters the inlet ends of the tubes, flow up through the tubesheet, and circulates around the U-shaped tubes which extend within the secondary side of the generator. As it circulates, this water transfers its heat through the walls of the tubes to the non-radioactive feedwater flowing through the secondary side of the generator, thereby converting the feedwater into non-radioactive steam. This steam in turn powers the turbines of an electric generator. After the water from the reactor circulates through the U-shaped tubes, it flows back through the tubesheet, through the outlets of the U-shaped tubes, and into the outlet section of the primary side where it is recirculated back to the nuclear reactor.
The walls of the heat exchange tubes in such nuclear steam generators can suffer a number of different forms of corrosion degradation, including denting, stress corrosion cracking, intragranular attack, and pitting. In situ examination of the tubes within these generators has revealed that most of this corrosion degradation occurs in what are known as the crevice regions of the generator. The principal crevice region for each of these U-shaped tubes is the annular space between the heat exchange tube and the bore in the tubesheet through which the tube extends. Corrosive sludge tends to collect within this annular crevice from the effect of gravity. Moreover, the relatively poor hydraulic circulation of the water in this region tends to maintain the sludge in this annular crevice, and to create localized "hot spots" in the tubes adjacent the sludge. The heat radiating from these "hot spots" acts as a powerful catalyst in causing the exterior walls of the heat exchange tubes to chemically combine with the corrosive chemicals in the sludge.
While most nuclear steam generators include blow-down systems for periodically sweeping the sludge out of the generator vessel with a jet of water, the sludges in the tubesheet crevice regions are not easily swept away by the hydraulic currents produced by such systems. Despite the fact that the heat exchange tubes of such generators are typically formed from corrosion-resistant Inconel.RTM. stainless steel, the combination of the localized regions of heat and concentrated corrosive sludge can ultimately corrode the walls of the heat exchange tubes until they crack. When this happens, the tubes begin to leak radioactive water from the primary side into the secondary side of the generator. This, in turn, results in radioactively contaminated steam flowing to the generators of the power plant.
In order to prevent such corrosion and tube cracking from occurring, various processes have been developed for radially expanding the sections of the tubes extending through the tubesheet so as to eliminate, or at least minimize, the annular space between the bores in the tubesheet and the heat exchange tubes. Such radial expansions may be implemented by hydraulic mandrels capable of applying fluid pressures of nearly 10,000 psi across selected sections of the tubes, or by cold-rolling tools which utilize pitched, tapered rollers capable of screwing into the open ends of the tubes and widening them until their outer walls engage the inner walls of the surrounding bore in the tubesheet. Unfortunately, such tube expansions create tensile stresses throughout the walls of the tubes in the tubesheet region which render them more susceptible to corrosion, thereby partially defeating the purpose of the tube expansion.
Because the metal around the inner diameter of the tube is expanded a relatively greater amount than the metal forming the outer diameter of the tube, most of the tensile stress caused by such radial expansions is concentrated in the inner wall of the tube. In order to relieve this tensile stress, rotopeening devices have been developed which are capable of peening and stress relieving the inner tube walls. However, most of these devices cannot be used to peen the heat exchange tubes located in the periphery of the tubesheet due to the fact that the open ends of these tubes are directly adjacent to the inwardly curving interior wall of the bowl-shaped primary side of the nuclear steam generator. To appreciate the spatial limitations imposed by the bowl and the tubesheet geometry in such generators, reference is made to the various figures of U.S. Pat. No. 4,262,402 (assigned to the Westinghouse Electric Corporation), which illustrate a side, cross-sectional view of the tubesheet and bowl-shaped primary side of such nuclear steam generators.
So far as applicant knows, only one rotopeening device has been specifically designed for use in these peripheral tubesheet regions. This device is described and claimed in U.S. patent application Ser. No. 731,241 filed May 7, 1985, now U.S. Pat. No. 4,616,496 by Phillip J. Hawkins and assigned to the Westinghouse Electric Corporation. Generally, this rotopeening device utilizes a rotopeening spindle having a flexible housing formed from a plurality of segments interconnected by means of two-way dogleg joints. A plurality of peening flappers are mounted on a flexible mandrel which in turn is rotatably mounted within the spindle housing. The use of such a flexible housing and flexible mandrel renders the entire spindle flexible. If rigid rotopeening spindles were utilized, the inwardly directed curvature of the bowl-shaped wall of the primary side of the generator would mechanically interfere with the alignement and insertion of the spindle into the open end of these peripherally located tubes. However, because the spindle or spindles used are flexible, these spindles can be bent to conform with the curvature of the bowl-shaped walls, and easily aligned and inserted into the open ends of such peripheral tubes. Unfortunately, this tool possesses one substantial shortcoming--it would be difficult, if not possible, to remotely insert the flexible spindles of this tool into the open ends of peripherally located tubes by means of known robotic devices, such as the ROSA developed and patented by the Westinghouse Electric Corporation. The flexibility of such spindles causes them to assume a naturally curved or "slumped" orientation when supported only at their bottom portions, thereby making it very difficult to insert the upper ends of these spindles into the open ends of the tubes to be rotopeened. And even if this were accomplished, other problems would arise if such a robotic arm were simply moved vertically in an attempt to insert the entire length of the flexible spindle into the tube. Specifically, the flexibility of the spindle, coupled with its curved or slumped attitude, would cause a vertical force component generated by the robotic arm to be transmitted out the side of this flexible spindle, thereby merely folding and breaking it, rather than threading it through the tube. In short, the problem to be solved is somewhat like attempting to thread a length of wet spaghetti through the bottom end of a vertically oriented tube glued to the inside wall of a fishbowl by holding the bottom end of the length of spaghetti and pushing up. This is a substantial shortcoming, since the radioactive environment present in the primary sides of "hot", in-service nuclear steam generators renders it very desirable to use robotics, rather than human operators, to implement all stages of a rotopeening process.
Clearly, there is a need for a peening apparatus capable of quickly, reliably and remotely peening the inner walls of the heat exchange tubes mounted around the periphery of the tubesheet of "hot" nuclear steam generators. Ideally, such an apparatus would be mechanically uncomplicated, and easily positionable into the open ends of the most remotely located tubes with pre-existing robotic devices, such as the ROSA.